The invention relates to dense silicon carbide bodies. More particularly, the invention relates to a method of forming silicon carbide articles, particularly those of complex shape.
Silicon carbide is not readily amenable to the more conventional cold pressing and sintering processes because the silicon carbide particles will not sinter and densify as will many other refractory materials. However, it has been found that by incorporating minor amounts of certain metals, e.g. boron or aluminum, in the silicon carbide powder, relatively dense silicon carbide bodies can in fact be fabricated by cold pressing a green form followed by sintering. An inherent characteristic of this approach is that the green preform shrinks in size on the order of 15 to 20 percent thus making it impractical, if not impossible, to accurately control the dimensions of the finished article.
Another fabrication technique for forming silicon carbide is the hot-pressing method, such as that described by R. A. Alliegro et al., Journal of the American Ceramics Society, 39, page 386 (1965) or by Weaver et al. in U.S. Pat. No. 3,836,673. In this approach powdered silicon carbide is loaded into a graphite mold and hot-pressed. Hot-pressing produces silicon carbide articles of close to theoretical density and accurate, predetermined dimensions. The only technical shortcoming of the hot-pressing method is that the articles must be of relatively simple geometry. Complex shapes such as a silicon carbide turbine engine rotor, or even the rotor blades, cannot practically be made via the hot-pressing route without final machining.
A third forming method is the so-called reaction sintering or self-bonding process. In general, a body or preform of silicon carbide powder or silicon carbide and a carbonaceous material is made by cold pressing, extruding, isostatic pressing, or the like. The preform is then contacted with silicon metal or silicon vapors at, for example 1700.degree. C, which causes the silicon to enter the interstices of the preform. If carbon is present the silicon will further react with the carbon to form additional silicon carbide. This general method is discussed in some detail by C. W. Forrest et al., in Special Ceramics, 5, page 99 (1972). Among the many variables studied by these workers was the effect on mechanical strength of the particle size of the starting silicon carbide powder. They found that particle size had a profound effect on cross-bending strength down to a particle size of about 100 microns but below that size the effect on strength was much less pronounced. Several permutations of the reaction sintering process have been described in the patent literature. J. C. Andersen in U.S. Pat. No. 2,938,807 describes the forming of dense silicon carbide bodies containing less than 5% free silicon by cold forming a uniform mixture of silicon carbide powder, a carbonaceous material, and a carbonizable material which functions as a temporary binder. The green form is then heat treated to carbonize the carbonizable material followed by a heat treatment at about 2250.degree. C in the presence of silicon metal, resulting in the in situ formation of silicon carbide in the pores of the original preform. Another reaction sintering method is disclosed by K. M. Taylor in U.S. Pat. No. 3,205,043 wherein a green silicon carbide preform is cold pressed from silicon carbide powder containing a small quantity of temporary organic binder, for example a phenolic resin, fired at 2300.degree. C to remove the temporary binder and to recrystallize the silicon carbide. The resulting porous structure is then impregnated with a carbonizable material such as a furfuryl compound or a phenol aldehyde resin and then heat treated to carbonize the organic materials. This latter step is repeated until the carbon content of the silicon carbide preform is 85 to 95 percent of what is needed to react with silicon to fill essentially all of the pores therein. The carbon containing silicon carbide structure is then contacted with silicon metal at about 2200.degree. C. The silicon penetrates the structure and reacts with the carbon to form silicon carbide. According to P. Popper in U.S. Pat. No. 3,275,722 dense self-bonded bodies of silicon carbide are formed by preforming a mixture of silicon carbide powder, powdered carbon, and a temporary binder. The preform is then heated at 1500.degree. C in a vacuum in the presence of silicon vapor, which eliminates the temporary binder and causes the silicon vapor to react with the carbon in the interstices of the silicon carbide-carbon preform. C. W. Forrest describes still another reaction sintering process in U.S. Pat. No. 3,495,939. Forrest forms a porous, coherent body of powdered silicon carbide and carbon, heats the body at from 1600.degree. to 1700.degree. C in an environment of silicon monoxide vapor which increases the surface porosity of the body, and while heating contacts one end of the body with molten silicon. Capillarity draws the molten silicon into the pores of the body forming silicon carbide therein. Still another variant on the reaction sintering approach is that described by U.S. Pat. No. 3,778,231 which is a process for forming sintered silicon nitride articles. In this process, powdered silicon metal is first sintered at 1200.degree. to 1400.degree. C in a nonnitrogen atmosphere, to form a blank or billet which is then machined to the desired shape. The silicon preform is then nitrided by exposure thereof to a nitrogen atmosphere at a temperature of 1180.degree. to 1350.degree. C.